This invention relates to spinnerette assemblies for forming hollow fibers. It particularly relates to an improved spinnerette for more efficient and precise production of multi-component hollow fibers.
It is well known to use various hollow fibers, generally made of one or more polymer materials, for various applications. For example, hollow fibers are used in carpets, as fill materials for pillows, as insulation materials for blankets and garments, and as membranes for gas separation, blood dialysis, purification of water, and other filtering applications. For membrane applications, the hollow fibers may be composed of a single component or a plurality of components, such as a hollow structured core with a sheath disposed around the core acting as a separating layer. The fibers can be bundled together and disposed in a tubular housing to provide a separation device known as a permeator. Ordinarily, the hollow fibers are relatively small, having a diameter on the order of 30 to 1000 micrometers. Accordingly, the apparatus and method for manufacturing hollow fibers must be very precise to be able to control the diameter of the fiber, and the concentricity of the core and sheath around the bore.
Numerous spinning assemblies have been devised for the production of single-component hollow fibers and multi-component hollow fibers of the sheath/core type. Particularly, devices have been proposed for ensuring uniform supply of the fiber-forming fluid or fluids to the orifices of a spinnerette with the object of producing hollow fibers identical in diameter, composition, and concentricity. These spinnerettes use a means for supplying the bore fluid positioned in the spinning orifice for forming the hollow fiber. Usually, a tube or needle is used for this purpose and a gaseous or liquid fluid is injected from the tube, thus forming the bore of the fiber as it is being extruded from the spinnerette orifice. For melt spinning, the nascent fiber can be solidified by cooling in a gaseous or liquid cooling fluid. For solution spinning, the nascent fiber can be solidified by evaporation of the solvent or by contacting the fiber with a solvent-extracting liquid that results in coagulation of the polymer solution(s) to form the fiber wall.
A typical spinning assembly, such as described in U.K. Patent No. 830,441 for a multi-component fiber, comprises a front and back plate spaced apart from but facing each other so as to provide a liquid channel there between. The front plate is provided with an extrusion orifice therethrough, and at least one of the plates, on the side facing the other plate, is provided with a plateau-type protrusion so as to constrict the liquid channel in a region surrounding the extrusion orifice entrance and, thus, cause the stream of the sheath-forming material to converge substantially radially towards the orifice entrance. A tube is positioned in the orifice entrance to supply the bore fluid. However, a continuing problem is the uniform supply of core- and sheath-forming material during the formation of the multi-component fiber. Most spinnerettes of this type are made largely by hand, one at a time. As a result, parts made for one spinnerette will not always fit another spinnerette. When parts are not interchangeable, any damage to one part of the spinnerette assembly may render the entire assembly useless. In assembling or cleaning conventional bicomponent or hollow-fiber spinnerettes, it is very easy to slightly bend the fluid-injection tube or needle, such that it is off center of the spinning orifice. When this happens, the spinnerette cannot be used until repaired.
Another related problem of conventional spinnerettes for multi-component hollow fibers is that the sheath and core of the hollow fiber are not concentric. Concentricity of the sheath and core are important to obtain uniform fibers. Concentricity of the sheath and core was customarily obtained by adjustment of metering surfaces to regulate (meter) the flow of the polymers. The metering surfaces are produced by closely machining two surfaces so as to produce a narrow opening which will effectively meter polymer at a uniform pressure and rate as it is being extruded. U.S. Pat. No. 3,458,615 discloses a method for maintaining sheath/core concentricity by circumferential metering of the polymer sheath to be extruded about the polymeric core involving the creation of an annular wedged-shaped flow of sheath polymer around the axially-contained fluid stream.
In order to maintain the concentricity of fiber diameter and the bore diameter, other spinnerettes have been provided with members for centering the tube and the bore of the spinnerette plate. For example, U.S. Pat. No. 4,493,629 describes a modular spinnerette assembly fitted with multiple screws threaded through the spinnerette plate to center the tube and orifice of the spinnerette. These adjusting screws are unreliable and are prone to error when the spinnerette is disassembled, cleaned and then reassembled. Many devices have been proposed for ensuring uniform supply of homogenous sheath-forming liquid to the orifices of a multi-orifice spinnerette with the object of producing multiple hollow fibers with concentric layers, identical in denier and other characteristics. Such devices usually involve variations in the diameter or the location of orifices and single or multiple spinnerette plates. Lack of concentricity and uniformity remain a problem in the manufacture of such single-component and multi-component fibers.
Another problem with existing spinnerettes is the ability to deliver the polymer fluid or fluids uniformly around the tube or needle within the spinnerette. U.S. Pat. No. 5,320,512 discloses a spinneret that has a plurality of discrete material passages formed around the needle to deliver the polymer fluid around the needle. The polymer fluid from these individual passages must converge and meld together to form a singular annular flow around the tube or needle as the polymer fluid traverses through the main polymer fluid passage. If complete melding is not attained, seams may develop down the length of the fiber at the interfaces where the individual flows did not fully converge.
Therefore, it would be desirable to have a spinnerette design which would permit the production of concentric and uniform fibers without the risk of seaming.
The present invention provides an improved spinnerette for the production of hollow fibers.
It is an object of the invention to overcome the limitations of conventional spinnerettes.
It is another object of the invention to reduce imperfections in hollow fibers.
It is another object of the invention to extend hollow-fiber production run times.
It is another object of the invention to reduce the time for spinnerette maintenance.
It is another object of the invention to simplify spinnerette fabrication.
It is another object of the invention to produce high quality composite fibers having one or more sheath layers in an efficient manner.
To achieve these objects, a first aspect of the invention is a spinnerette assembly for forming a composite hollow fiber comprising:
at least one extrusion orifice formed in said spinnerette assembly;
a hollow needle extending through each said extrusion orifice in a concentric manner to define an annular passage around said needle in said extrusion orifice;
a bore forming fluid passage communicating with an interior of each said needle;
at least one core forming material passage formed in said spinnerette assembly, wherein each said core forming material passage comprises a core forming material inlet port extending from a surface of said assembly to an interior of said assembly and at least one transverse passage extending from said core forming material port to each said annular passage, wherein a portion of said transverse passage entirely surrounds each said needle in a continuous manner; and at least one sheath forming material passage, wherein each said sheath forming material passage comprises a sheath forming material port extending from a surface of said assembly to each said annular passage.
A second aspect of the invention is a method of forming a composite hollow fiber comprising the steps of:
delivering a core forming material to each annular passage in the spinnerette assembly, said core forming material entering the spinnerette assembly through one or more core forming material inlet ports and passing through the interior of said assembly to a transverse passage, a portion of said transverse passage entirely surrounding each needle in a continuous manner, and through an annular passage in communication with an extrusion orifice, and
delivering at least one sheath forming material concentrically around the core forming material as it traverses through each said annular passage,
extruding the layered core forming material and at least one sheath forming material through the extrusion orifice and around each said needle,
injecting a bore forming fluid into each needle to thereby provide a layered composite fiber comprising a bore forming fluid, a core forming material, and a sheath forming material as it exits the spinnerette assembly through the extrusion orifice,
optionally passing the nascent extruded hollow fiber through an air gap, and
solidifying the hollow fiber by cooling, solvent evaporation, or solvent extraction.